Print and transport belt synchronizer

ABSTRACT

A high-speed serial printer including a continuously rotating print drum having characters on its periphery, a hammer and carrier, and a belt for transporting the hammer and carrier parallel to the axis of drum along a line of print. A control system, including two timing discs, operates in synchronism with the drum. The first disc is indicative of the angular position of the drum and the second disc is to signal the hammer carrier to engage the transport belt at the start of each line of print. A double-ended transducer responds to the two timing discs to provide the appropriate signals for the printer.

15] 3,667,383 1451 June6, 1972 United States Patent Mack et a].

3,117,514 1/1964 Doersam,.lr.................

Foley [54] PRINT AND TRANSPORT BELT SYNCHRONIZER [72] Inventors: RonaldR. Mack, Plymouth; Nicholas Konm S m y m m d .& u no or... m m cm m m WM$4 a Em mi dur, Jr., Plymouth Township, both of Mich.; Janis A.Mitchell, Paris, France [73] Assignee: Burroughs Corporation, Detroit,Mich.

print. A double-ended transducer responds to the two timin discs toprovide the appropriate signals for the printer.

9 Clairm, 5 Drawing Flgures x We 93 Wm DKR PATENTEDJUN 6 I972 SHEET 10F3 FIG.

DIFFERENTIAL AMPLIFIER INVENTO S RONALD H. MACK,

NICHOLAS KONDUR,Jr. 8. JAMES A. MYITCHELL AGENT PATENTEUJun 6 m2 SHEET 2BF 3 A mm r" m d TK. 3E T NC EAUC N VMDT E m mm m H K A DS LAE Mu 7 H YB 1 NQE PATENTEDJun s 1912 SHEET 3 OF 3 FIG.4.

R HM mK L 16 E N UH EADC w Nw Hm D A S MAS 6N t OM l-l w m m Na In Hlb.H q

PRINT AND TRANSPORT BELT SYNCHRONIZER BACKGROUND OF THE INVENTION Thisinvention relates to high-speed serial printers and, more particularly,to the control system for such a printer.

High-speed printers find many applications as, for example, inconnection with electronic computers, electronic desk calculators andthe like. Many of these high-speed printers utilize continuously movingtype faces carried on the surface of rotating drums, chains, or belts.The prior art high-speed printers employing continuously moving typeprovide one of two printing hammer arrangements; a row of printinghammers, one for each character position along the line of print, or asingle hammer which is transported along the line of print from onecharacter position to the next.

In the latter type of highspeed printer, the transport mechanism whichmoves the hammer along the line of print may be a threaded shaft, as iscommon in strip printers, or a toothed belt. A detailed explanation ofthe operation of a high-speed serial printer employing a toothed belttransport mechanism may be seen, for example, in US. Pat. No. 3,472,352to Nicholas Kondur, Jr., assigned to the assignee of 'the presentinvention.

In high-speed printers using a single print hammer and a rotating drumin conjunction with a computer, one or more types of timing signals maybe necessary to coordinate drum rotation, hammer movement and flow ofdata from the central processor. For example, signals (1) to initiatethe movement of the print hammer and carrier at the start of each lineof print, (2) to indicate the position of the print hammer along theline of print, and (3) to cause the hammer to print, i.e., force thepaper against the drum, may be necessary for the correct operation of aprinter.

In the prior art devices, such as the aforementioned patent to Kondur,the central processor or data source signals the initiation of each lineof print. This causes the drum to start rotating and the print hammerand its associated hardware to engage the transport belt. However, ifthe angular position of the drum varies slightly the synchronism betweenthe drum the print hammer may be lost.

In high-speed printers of the type described there must besynchronization between the drum and print hammer to cause the correctcharacter to be printed. For this purpose the prior art devices havefavored a rotating timing disc having teeth thereon corresponding to thecharacters on the periphery of the drum. The teeth cooperate with atransducer to generate pulses indicative of the angular position of thedrum. These pulses serve as one input to a comparator. The other inputto the comparator is from the central processor and represents thecharacter to be printed. When these two inputs to the comparator match,the appropriate signal is given by the comparator and the printinghammer is actuated to print the character. After a character is printed,an irregular spacing of one tooth may be used to reset the comparator.This type of timing apparatus may be seen, for example, in US. Pat. No.3,117,514, to C. Doersam, Jr. However, in this arrangement, anydeviation in the speed of the rotating drum may defeat the precisetiming required to interpret the irregular teeth spac- To overcome thisproblem, the prior art devices have suggested two alternatives; eithertwo timing discs, each having its own transducer or a single timing dischaving two transducers. An example of the apparatus using a singletiming disc and two transducers may be seen in US. Pat. No. 3,291,909,to Clark et al.

As can be appreciated from a review of these prior art devices, the costof these devices may be reduced by eliminating duplicate circuitry, suchas two transducers, and by permitting the same timing pulses to performplural functions without a loss of synchronism.

SUMMARY OF THE INVENTION Accordingly, with these prior art problems inmind, the invention contemplates. the solution to these problems byproviding a new and improved serial printer control system includingmore reliable synchronization.

In addition, it is an object of the present invention to provide a newdouble-ended transducer for responding to multiple timing means.

It is a further object of this invention to maintain synchronism in ahigh-speed drum printer regardless of the initial position of the printdrum.

It is yet another object of this invention to eliminate the timingproblems which arise with the use of irregular spacing of the timingteeth for resetting a comparator in a high-speed printing operation.

These and other objects are accomplished in a high-speed serial printerincluding two timing discs driven in synchronism with the rotating drumof the printer. The first timing disc generates pulses to engage theprint hammer with a hammer transport belt and the second timing discgenerates pulses indicative of the location of the character on theperiphery of the drum. A double-ended transducer responds to both timindiscs.

BRIEF DESCRIPTION OF THE DRAWINGS The foregoing objects and advantagesof our invention, together with other advantages which may be attainedby its use, will be apparent from the following detailed description ofthe invention when read in conjunction with the drawings. In thedrawings, wherein like numerals refer to the same feature:

FIG. 1 is an overall perspective view of a high-speed serial printerincluding the features of our invention;

FIG. 2 is an illustration of the two timing discs, the doubleendedtransducer, and associated control circuitry;

FIG. 3 is a block diagram of the logic circuitry for demodulating thesignals from the double-ended transducer;

FIG. 4, comprising FIGS. 4A 4C, is a schematic diagram of part of thelogic circuitry of FIG. 3; and

FIG. 5, comprising FIGS. 5A 5G, is a timing diagram illustrating theoperation of the control circuitry.

DESCRIPTION OF THE PREFERRED EMBODIMENT and belt 31. A carrier 25 ismounted forwardly of the drum for carrying hammer is along a print line27 parallel to the axes of both drum 13 and shaft 23. I

The drum is driven by electric motor 29 through a toothed drive belt 31.The drum is driven continuously by the motor to move print elements ortype on the periphery of the drum past the print line 27. The printelements, which may include letters, numerals and any other symbols(hereinafter referred to as characters), are arranged on the peripheryof the drum in rows which are parallel to the drum axis and in columnswhich extend part way around the drum. The columns of type extend partway to allow time for the hammer 15 to be retracted after a character isprinted. The printing type are arranged such that the characters of eachcolumn are in a predetermined sequence and the sequence is repeatedtwice in each column with a dwell or blank area between the sequences.Furthermore the print elements for each column are oriented such that ata given time the same character appears in each column along the line ofprint 27.

I-Iammer 15 is pivotably mounted on carrier 25. The hammer isconventionallybiased away from drum l3 and solenoid 33 operates to firethe hammer forward against paper 35 and drum 13. The characters in aline of print are printed serially from right to left.

v Shaft 37, located at the right side of the printer 11 andperpendicular to shaft 23, is driven by motor 29 in synchronism withdrum 13. Drive pulley 39 is mounted on the front end of shaft 37 andguide pulley 41 is mounted at the left side of the printer. Each pulleyhas its axis of rotation horizontally disposed. Transport belt 17, anendless belt, is passed around the pulleys 39 and 41 and is driven in acounterclockwise direction by the rotation of shaft 37 which drivespulley 39.

In the aforementioned Kondur, Jr. patent, there is a detaileddescription of the clutch means for engaging the carrier with thetransport belt. In the present invention the location of such a clutchmeans in designated 43. The clutch means 43, which includes a solenoid45 for activating the clutch (FIG. 2), is attached to carrier 25. Whenthe solenoid 45 is activated the clutch means 43 engages the teeth ofthe belt 17 and the hammer and carrier are transported from right toleft along the line of print 27. Spring 47 urges the carrier 25 to arest position which is defined as the extreme right side of .the line ofprint 27.

Mounted on one end of shaft 23 is a character timing disc 19 having aseries of flux producing points, such as teeth 49, on its periphery. Theflux producing points or teeth correspond to the location of the varioustype elements on the periphery of the drum 13. Since there are two setsof type elements on the periphery of the drum 13, which are separated byblank areas or dwells, there are two corresponding sets of teeth 49 onthe'periphery of disc 19 and two corresponding dwells. The use of twosets of type elements or characters will be more fully explainedhereinafter. A bevel gear 51 is mounted on the right end of shaft 23.Shaft 53 is mounted on the printer 1 l to the right of shaft 23 andperpendicular thereto. (Shaft 53 is parallel to shaft 37.) Bevel gear 55is 'mounted on the front end of shaft 53 and is oriented to mesh withbevel gear 51. Transport timing or belt engage disc 21 is rotatablymounted on the rear of shaft 53. Disc 21 has a single flux producingpoint, such as stud 63, thereon. Therefore, when motor 29 rotates drum13, character disc 19 and transport disc 21 rotate in synchronismtherewith.

A double-ended transducer 57 is located intermediate the two discs 19and 21. The front end 59 of the double-ended transducer responds to theflux producing points or teeth 49 on the character disc 19. The rear end61 of the transducer 57 responds to the flux producing point or stud 63on the second disc 21.

The structure of the double-ended transducer can be understood byreferring to FIG. 2. The front and rear ends 59, 61, respectively, oftransducer 57 are two pole pieces of ferromagnetic material. A permanentmagnet 65, which may be a ceramic magnet, is disposed between and incontact with both pole pieces 59, 61. A coil 67 is wound around one polepiece (e.g., 59) and then around the other pole piece (e.g., 61) in thesame direction. Hence the two ends of the transducer are wound inseries. One end of coil 67 is grounded and the other end isconnected toa control circuit 69 via lead 71. Altematively, both ends of coil 67could be connected to the control circuit 69 as though the coil were thesecondary of a transfonner.

The control circuit 69 includes a phase demodulator 73, a two inputcomparator 75, a data source 77, a three input AND-gate 79 and an SCR(Silicon Controlled Rectifier) 81. The output of phase demodulator 73 isconnected as one input to comparator 75, via lead 83, and as one inputto three-input AND-gate 79, via lead 85. The output of data source 77 isthe Second input to comparator 75, via lead 87, and the second input toAND-gate 79 via lead 89.

When carrier 25 is in the rest or home" position, as defined previously,magnetic shunt 91 is prevented from entering the magnetic path betweenmagnet 93 and magnetic switch 95 and switch 95 is closed. Lead 96connects switch 95 to the third input of AND-gate 79. Therefore, onlywhen switch 95'is closed can AND-gate 79 be enabled. Once carrier 25starts to move along the line of print 27 the shunt 91 moves betweenmagnet 93 and switch 95 opening the switch 95 and inhibiting AND-gate79. A fuller explanation of the operation of the printer, including thecooperation between carrier 25 and shunt 91, may be found in US. Pat.No. 3,472,352, Kondur, Jr., assigned to the assignee of the presentinvention.

The output of AND-gate 79 is connected to the gate of SCR 8]. Thecathode of the SCR is grounded and the output of the SCR is taken fromthe anode and connected to a tum off or timer 97. The output of timer 97is connected via lead 98 to clutch-engage solenoid 45. When AND-gate 79is enabled the output therefrom fires SCR 81. The output from the SCRactivates the clutch solenoid 45 via lead 98 to permit the belt clutch43 to engage the toothed transport belt 17. Since one input to AND-gate79 occurs only when switch 95 is closed, the clutch can only beenergized when the carrier 25 is at the rest position. At the completionof a line of print, conventional timer 97 will deactivate SCR 81 andde-energize solenoid 45. Clutch 43 will release the toothed belt 17 andspring 47 will return the carrier to its rest position. v

Comparator is a conventional logic device which provides an output onlywhen the two inputs agree or match. One

input to comparator 75 is the character to be printed (from data source77 via lead 87) and the other input is the character on the drum inposition to be printed (from teeth 49 via demodulator 73 and lead83).'The output of comparator 75 is connected via lead 99 to one inputof a two input AND- gate 101. The other input of AND-gate 101 has aninverted leg 111. The output of switch is also connected via lead 1 13to this inverted leg 111 of the AND-gate 101. Hence, AND-gate 101functions to indicate the presence of a signal from the comparator 75and, an open switch 95. (The absence of a closed switch 95.) Recallingthe earlier discussion, open switch 95 indicates that the carrier is notat rest but has engaged transport belt 17 and is moving along the lineof print 27. When gate 101 is enabled, itsoutput, via lead 115,activates hammer solenoid 33 to fire the print hammer 15.,

Referring next to FIG. 3, the logic circuitry of the phase demodulator73 will be explained. The demodulation circuit 73 includes adifferential amplifier 117, having two outputs designated A and B. The Aoutput of the differential amplifier is connected to an amplifier ordriver 119 and the driver is connected to an AC coupled timing circuit121. The output of timing circuit 121 isconnected as one input to eachof the AND-gates 123 and 125.

The B output of the differential amplifier 117 is connected to a firstinverter 127. The output of inverter 127 is connected through a diode129, which is poled with its anode toward inverter 127, to a secondinverter 131. One terminal of a capacitor 133 is connected to thejunction 130 of the cathode of diode 129 and inverter 131.'The otherterminal of the capacitor is biased negative. The second inverter 131has its output connected to the other input of AND-gate 123. The outputof inverter 131 also serves as an input to a third inverter 135. Theoutput of inverter 135 is the second input to AND-gate 125.

Since a magnetic transducer responds to the rate of change ofreluctance, the output voltage of the magnetic transducer is the firstderivative of the amount of activating mass in front of the pole pieceof the transducer. When a character pulse tooth 49 from character disc19 passes the front pole piece 59, the slope or rate of change of thevoltage output of the transducer is positive on the leading edge, zeroin the center, and negative on the trailing edge. When the belt engagestud 63 passes the rear pole piece 61 the reverse output waveform occurssince the stud passes the opposite pole piece of the transducer and thepole pieces are wound in series. The output is negative on the leadingedge, zero in the center, and positive on the trailing edge. This isseen from Lenzs Law for magnetically inducing current into a coil.

The various circuits employed in the logic circuit of FIG. 3 are wellknown in the art. However, in order to provide a complete disclosure,illustrative schematic circuits are shown. FIG. 4A is representative ofthe differential amplifier 1 17. The two inputs may be obtained from thetransducer 57 using a conventional transformer. (Alternatively both endsof coil 67 could be used instead of grounding one side of the coil.) Thetwo PNPotransistors have their bases connected through two equalresistors R (1,000 ohms each). At the junction of the two resistors anegative bias is provided by voltage V, (V l4 volts). The emitters ofthe transistors are connected together and a bias is provided to theemitters via voltage V (V 28 volts) connected through resistor R (5,100ohms). The collector of each transistor is connected to ground through aresistor R; (6,200 ohms) and the outputs of the amplifier are taken fromthe collector of each transistor.

Amplifier or driver 1 19 is noninverting merely shapes the A output ofthe differential amplifier and does not contribute to the logicconsiderations.

FIG. 48 illustrates a typical inverter such as inverters 127, 131, and135. The input to the inverter passes through resistor R (30 kilohms)and then to the base of a PNP-transistor. Negative potential is providedat the junction of resistor R and the base of the transistor via voltageV through resistor R (270 kilohms). The emitter of the transistor isbiased negative (V,) and the collector of the transistor is connected toground through resistor R The output is taken from the collector.

Referring to FIG. 4C a representative schematic circuit for the ACcoupled timing circuit 121 will be explained. The input to the timingcircuit is connected to one terminal of a coupling capacitor C (470picofarads). At this input a negative bias is provided from voltage Vcoupled through resistor R, (2,000 ohms). The other terminal of thecapacitor is coupled through an equal resistor R to the base of aPNP-transistor. The junction of the capacitor and the second resistor Ris connected via resistor R (47 kilohms) to ground. The emitter of thetransistor is connected to a source of negative potential V and thecollector of the transistor is coupled via resistor R to ground. Theoutput of the timing circuit is taken from the collector. I I

Referring to FIGS. 2, 3, and 5, the timing sequence of the controlcircuitry will be explained. FIG. 5A illustrates the output waveform ofthe transducer 57. FIG. 5B represents the'A output of the differentialamplifier 117. FIG. 5C represents the output signals from the timingcircuit 121 and, as can be seen by comparison with FIGS. 5A and 5B, thetiming circuit generates an output signal on the negative-going portionof each output pulse from the A output of the differential amplifier.FIG. 5D represents the B output of the differential amplifier 117 andFIGS. 5E, 50, and SF represent the pulse trains taken from the output ofinverters 127, 131, and 135, respectively. When an inverter (e.g.,inverter 135) is referred to as high its output will enable the circuitto which it is connected (e.g., AND-gate 125) and when an invertor isreferred to as low" its output will inhibit the gate to which it isconnected.

In the quiescent state t, the differential amplifier 117 is biased tohold the output of amplifier or driver 119 high and inverter 127 low.Amplifier 1 19 being high" holds the output of timing circuit 121 lowwhich inhibits both AND-gates 123 and 125. Inverter 127 low holdsinverter 131 high, and, inverter 131 high holds inverter 135 low. As acharacter tooth 49 starts past pole piece 59 of transducer 57, theoutput of the transducer (FIG. 5A) causes an output from thedifferential amplifier (FIGS. 5B, 5D). This output causes inverter 127(FIG. SE) to go high", capacitor 133 to charge, inverter 131 (FIG. 5G)to go low, and inverter 135 (FIG. 5F) to go high. This is shown, forexample, as time I in FIG. 5.

As the center of this tooth passes the transducer, at time inverter 127goes low", but the charge on the capacitor 133 causes a delay whichholds inverter 131 low and inverter 131 low holds inverter 135 high.With the passing of the center of the tooth the output of amplifier ordriver 119 also goes "low. When the output of the driver goes low, ittriggers the AC coupled timing circuit 121. The timing circuit is aconventional circuit which responds to the negative-going edge of apulse to generate an output. Since the output of the timing circuit 121is used to sample or enable the gates 123 and 125 it is often referredto as a pulse sampling circuit."

At time when a pulse sample signal is generated by timing circuit 121,inverter 127 has just gone "low" but capacitor 133 will hold junction130, the input of inverter 131, high.

The presence of the high" at the input of inverter 131 will cause itsoutput to remain low" which in turn holds the output of inverter 135high. Since inverter 131 is low it inhibits AND-gate 123. This preventsa signal from being generated along lead to AND-gate 79 and inhibitsengaging the carrier 25 to the transport belt. However, since the outputof inverter 135 remains high", AND-gate 125 is enabled. This permits asignal to be passed via lead 83 to the comparator 75.

At time when capacitor 133 discharges, inverter 131 goes high andinverter 135 goes on. The logic circuitry is now in the prevents theoutput along lead 83 from serving as an input to the comparator 75. Butsince inverter 131 is high, AND-gate 123 is satisfied and the output onlead 85 permits the carrier 25 to engage the transport belt 17.

As the middle of the stud 63 passes the pole piece (1,, inverter 127will go high and charge capacitor 133. However, the timing circuit willnot trigger again'until a new tooth (or stud) passes a pole piece.Hence, the delay in discharging the capacitor until time i when thequiescent condition is reached, will not affect the output of gates 123or 125.

Additional aspects of the operation of this invention will now bedescribed. Assume that motor 29 is operating and, via belt 31, isdriving the drum 13. The first condition is to initiate a line of print.The carrier 25 is at the extreme right hand position, that 'is, at therest position. When this occurs the magnetic shunt 91 will not bebetween magnet 93 and switch 95; therefore, the switch contacts will beclosed." By virtue of switch 95 being closed an input will be presented,via lead 96 to one of the three inputs of AND-gate 79. An appropriatesignal will be presented from data source 77 via lead 89 to a secondinput of AND-gate 79. Since a line of print is to be initiated, asdistinguished from printing various characters,- there is no signal fromdata source 77 to comparator 75 on lead 87 at this time.

The rotation of shaft 23 by the motor 29 causes disc 19 to rotate andthe teeth 49 to pass pole piece 59 of double-ended transducer 57. Aseries of output pulses are generated similar to those shown in FIG. 5Abeginning at These pulses will each enable AND-gate 125, but, since oneinput is missing to comparator 75 (there is no input at lead 87) thehammer will not fire and no character will be printed. In addition, viain verted leg 111 of AND-gate 101, the hammer is precluded from firing.Belt engage disc 21 is also rotating, and, when belt engage stud 63passes the right pole piece 61 of transducer 57, a waveform similar tothat shown at times 1 -1 in FIG. 5A is generated. This will enableAND-gate 123 and a signal is sent via lead 85 to AND-gate 79. Since allthree inputs are now applied to AND-gate 79 the gate is enabled and thisfires the SCR 81. When SCR 81 fires, a signal via lead 98 energizes thebelt engage clutch solenoid 45 and the carrier 25 engages the belt 17.This starts the carrier 25 moving along the line of print 27 from rightto left.

Once the carrier 25 has engaged transport 17 and is travelling along theline of print the second condition, i.e., the characters to be printed,is communicated from the data source 77 via lead 87 to comparator 75. Asthe teeth 49 of demodulator 73. These pulses are sampled on thenegative'or trailing edge, and each pulse enables AND-gate 125 asexplained previously. Each time gate 125 is enabled, a signal is passedto comparator 75 via lead 83. When and only when the signals incomparator 75 match, an output is provided via lead 99 as one input toAND-gate 101. When the carrier 25 starts to move from the rest positionthe magnetic shunt 91 moves between magnet 93 and switch 95. Thisoperation was more fully described in the aforementioned Kondur patent.Moving the magnetic shunt between the magnet and the switch causes theswitch contacts to open and provides an open circuit along lead 113.This open circuit is converted via inhibit leg 111 at the second inputof AND gate'l01. Consequently, when the comparator 75 indicates theappropriate signal, via lead 99, AND-gate 101 is enabled and a signalvia lead 115 fires hammer solenoid 33 causing the hammer 15 to strikethe drum 13 and print the appropriate character along the line of print27 Switch 95 opens as the carrier moves from the rest position sinceshunt 91 enters the path between magnet 93 and the switch. Lead 96connects the output of switch 95 to one input of AND-gate 79, and, whenswitch 95 is open AND-gate 79 is inhibited. This prevents a second beltengage signal from being transmitted to the clutch solenoid 45 once thebelt has been engaged for a given line of print. As the drum 13continues to rotate, the stud 63 will again pass the right end 61 oftransducer 57. However, absent information from the data source 77 toAND-gate 79 requiring a new line of print and absent the closing ofswitch 95, this signal has no effect. As character disc 19 continues torotate, the second set of characters on the wheel pass through the printarea and, simultaneously, the second set of teeth 49 pass pole piece 59of transducer 57. Again the appropriate signals are sent through thephase demodulator to the comparator and, when the signals in comparator75 match, a second character is printed.

Thus it may be seen that when the carrier is at rest the print pulsesare inhibited via the closed circuit in switch 95 and the only pulsewhich can activate the system is a belt engage pulse generated by stud63 on disc'21. Similarly, once the carrier has started its travel, beltengage pulses are inhibited by the open switch 95 and only the printpulses can activate the system. At the conclusion of a line of print,based on the tim ing considerations of a given line, timer 97 turns offthe SCR 81 and de-energizes clutch solenoid'45to release the carrier 25"from the belt.l7. Spring 47 brings the carrier back to rest positionready to print a new line of print.

Various modifications of this system will be obvious to those skilled inthe art. For example, since there are two sets of type elements on drum13 and two sets of teeth 49 on disc 19, for

each revolution of the drum two characters may be printed. Since theprint engage pulse is used only once for a single line of print thespeed of disc 21 may be reduced without a loss of synchronism.Therefore, by the use of a 2:1 reduction bevel gears the speed of disc21 may be halved.

In addition, if the comparator 75 is not internally reset after eachcharacter is printed, the output via lead 83 may be used for thispurpose since the number of teeth 49 correspond to the number ofcharacters which could be printed in a given print position. Toaccomplish this, one of two alternatives is necessary. First, two studs63 could be used on disc 21. The studs would be l80 apart. Then, onestud 63 would pass the pole piece'6l after each set of character teeth49. Second, using only one stud 63, the bevel gears 51, 55 would berequired to have a l:2 speed increase. Therefore disc 21 would rotatetwice as fast as character disc 19.

Furthermore, pulses from stud 63 may be used as an input to data source77 to indicate the print positions along a line of print 27. Inaddition, stud 63 may be replaced by a slot and the direction of thewinding 67 on pole piece 61 would be reversed. The opposite polaritywaveform would still be obtained from disc 21.

Therefore, it will be appreciated that those skilled in the art may makevarious changes and modifications and still remain within the spirit andscope of this invention.

What is claimed is: 1. In a high-speed serial printer including a printdrum having characters thereon, said drum being rotatable about itsaxis, a print hammer mounted on a hammer carrier, said hammer carrierhaving a rest position, and means for continu: ously transporting saidcarrier along a line of print, said line of print being parallel to saidaxis, a control system comprising:

timing means coupled to said drum for synchronous rotation therewith, I

a single double-ended transducer having a first transducing means at oneend thereof and a second transducing means at the opposite end thereof,said first andsecond transducing means being responsive to said timingmeans for generating first and second signals respectively, meansresponsive to said first signal for engaging said hammer carrier withsaid transporting means, and

means responsive to said second signal for firing said print of saiddrum relarest position for enabling said engaging means and responsiveto said carrier being in other than said rest ing said engaging means.

3. The control system of claim 1 wherein said firing means position forinhibitincludes means responsive to said carrier being in said restposition for inhibiting said firing means.

4. The control system of claim 1 wherein said timing means includes: v

a character disc having flux producing points thereon corresponding tothe angular location of the characters on said drum, and an engage dischaving at least one flux producing point thereon. i 5. The controlcircuit of claim 1 wherein said first and second transducer meansincludes first and second magnetic pole pieces, respectively, and coilmeans wound on said pole pieces for connecting said first transducermeans in series with said second transducer means. 7 i 6. The controlsystem of claim 5 further including circuit means for differentiatingbetween said first and second signals. 7. The control circuit of claim 6wherein said circuit means includes:

a differential amplifier having an input coupled to said coil means,

logic means coupled to the output of said differential amplifier fordifferentiating between said first and second signals, and

means coupled to said logic means for transmitting said first signal tosaid engaging means and for transmitting said second signal to saidfiring means. 8. The control system of claim 4 wherein said timing meansfurther includes:

means for positioning said engage disc and enabling said at least oneflux producing point to pass in proximity to said first transducer meansfor generating said first signal, and means for positioning saidcharacter disc and enabling said flux producing points thereon to passin proximity to said second transducer means for generating said secondsignal. 7 9. The control system of claim 8 wherein said character discincludes at least one blank area devoid of flux producing points andwherein at least one of said positioning means includes means fororienting said discs with respect to one another and insuring that saidat least one flux producing point of said engage disc passes inproximity to said first transducer means only when said blank area ofsaid character disc passes in proximity to said second transducer means.

1. In a high-speed serial printer including a print drum havingcharacters thereon, said drum being rotatable about its axis, a printhammer mounted on a hammer carrier, said hammer carrier having a restposition, and means for continuously transporting said carrier along aline of print, said line of print being parallel to said axis, a controlsystem comprising: timing means coupled to said drum for synchronousrotation therewith, a single double-ended transducer having a firsttransducing means at one end thereof and a second transducing means atthe opposite end thereof, said first and second transducing means beingresponsive to said timing means for generating first and second signalsrespectively, means responsive to said first signal for engaging saidhammer carrier with said transporting means, and means responsive tosaid second signal for firing said print hammer at the correct angularposition of said drum relative to said line of print.
 2. The controlsystem of claim 1 wherein said engaging means includes means responsiveto said carrier being in said rest position for enabling said engagingmeans and responsive to said carrier being in other than said restposition for inhibiting said engaging means.
 3. The control system ofclaim 1 wherein said firing means includes means responsive to saidcarrier being in said rest position for inhibiting said firing means. 4.The control system of claim 1 wherein said timing means includes: acharacter disc having flux producing points thereon corresponding to theangular location of the characters on said drum, and an engage dischaving at least one flux producing point thereon.
 5. The control circuitof claim 1 wherein said first and second transducer means includes firstand second magnetic pole pieces, respectively, and coil means wound onsaid pole pieces for connecting said first transducer means in serieswith said second transducer means.
 6. The control system of claim 5further including circuit means for differentiating between said firstand second signals.
 7. The control circuit of claim 6 wherein saidcircuit means includes: a differential amplifier having an input coupledto said coil means, logic means coupled to the output of saiddifferential amplifier for differentiating between said first and secondsignals, and means coupled to said logic means for transmitting saidfirst signal to said engaging means and for transmitting said secondsignal to said firing means.
 8. The control system of claim 4 whereinsaid timing means further includes: means for positioning said engagedisc and enabling said at least one flux producing point to pass inproximity to said first transducer means for generating said firstsignal, and means for positioning said character disc and enabling saidflux producing points thereon to pass in proximity to said secondtransducer means for generating said second signal.
 9. The controlsystem of claim 8 wherein said character disc includes at least oneblank area devoid of flux producing points and wherein at least one ofsaid positioning means includes means for orienting said discs withrespect to one another and insuring that said at least one fluxproducing point of said engage disc passes in proximity to said firsttransducer means only when said blank area of said character disc passesin proximity to said second transducer means.